Method of processing radioactive liquid wastes containing radioactive ruthenium

ABSTRACT

A method of processing radioactive liquid wastes containing radioactive ruthenium by passing said wastes through a column packed with an adsorbent comprising a mixture of activated carbon with zinc and palladium powders is herein disclosed.

BACKGROUND OF THE INVENTION

A. Technical Field

The present invention relates to a method of processing radioactive liquid wastes containing radioactive ruthenium. More particularly, the invention relates to a method for processing such radioactive liquid wastes by using an adsorbent comprising a mixture of activated carbon with zinc and palladium powders.

B. Prior Art

Radioactive ruthenium (¹⁰⁶ Ru) in radioactive liquid wastes react with nitric acid present in the processing of said wastes to form various nitrosyl compounds. Since these compounds are dissolved in the form of various complex salts, one method is capable of removing only a specific compound, and other nitrosyl compounds are left unremoved.

While many radioactive substances are present in the liquid wastes that are discharged into oceans from spent fuel reprocessing plants after being subjected to a chemical treatment such as concentration by evaporation, settlement by coagulation, or ion exchange reaction, ¹⁰⁶ Ru accounts for more than half of such radioactive substances. However, experiments conducted so far have shown that ¹⁰⁶ Ru cannot be removed with high efficiency by settlement after coagulation or ion exchange. The technique of concentration by evaporation is more effective than these two methods but its effectiveness is reduced if the radioactive liquid wastes contain a large amount of ¹⁰⁶ Ru in an highly volatile chemical form.

Japanese Patent Public Disclosure No. 50698/1982 (Japanese Patent Application No. 126401/1980) shows a method of removing ¹⁰⁶ Ru from radioactive liquid wastes by passing it through a column packed with a mixture of metal powder and activated carbon. However, this prior art technique has yet to be improved in respect of its ability to remove ¹⁰⁶ Ru.

As will be clear from the above explanation, ¹⁰⁶ Ru is one of the nuclides that are most problematic in the processing of radioactive wastes, and the development of a safe and efficient method of removing ¹⁰⁶ Ru without causing environmental pollution is greatly needed.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method of processing radioactive liquid wastes containing ¹⁰⁶ Ru.

Another object of the present invention is to provide a method of processing radioactive liquid wastes containing ¹⁰⁶ Ru by passing said wastes through a column packed with an adsorbent comprising a mixture of activated carbon with zinc and palladium powders.

A further object of the present invention is to provide a method of regenerating a deactivated adsorbent comprising a mixture of activated carbon and zinc and palladium powders by washing the adsorbent with aqueous nitric acid or water.

Other object of the present invention and its advantages will become apparent by reading the following description.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, ¹⁰⁶ Ru can be removed from radioactive liquid wastes by passing it through a column packed with an adsorbent comprising a mixture of activated carbon with zinc and palladium powders.

A mixed adsorbent of zinc powder and activated carbon is conventionally used in removing ¹⁰⁶ Ru from radioactive liquid wastes. The method of the present invention is characterized by adding a small amount of palladium powder to this mixture for the purpose of achieving a more efficient removal of ¹⁰⁶ Ru. A column packed with the conventional mixture of zinc powder and activated carbon requires an optimum use pH of about 2 and cannot be used in the neutral or alkaline region for achieving best results in the removal of ¹⁰⁶ Ru. However, by adding a small amount of palladium to the mixture of zinc powder and activated carbon, the optimum pH range for ¹⁰⁶ Ru removal is extended to cover not only the acidic region but also the neutral and alkaline regions. Furthermore, the mixture of activated carbon with zinc and palladium powders has a higher efficiency of ¹⁰⁶ Ru removal than the simple mixture of activated carbon and zinc powder. A probable reason for this higher efficiency would be as follows: palladium having a positive standard potential enhances the electrochemical action between the activated carbon anode and the powdered zinc cathode, and various nitrosyl ruthenium compounds are oxidized into more easily removable chemical forms. In other words, the method of the present invention utilizes both the adsorbing action of activated carbon and the electrochemical action that occurs between the carbon palladium electrode and the zinc electrode in the liquid electrolyte (liquid wastes). As a result, the method of the invention is capable of removing nitrosyl ruthenium compounds that are difficult to eliminate by the conventional techniques, and this contributes to the increased ability of the invention to remove ¹⁰⁶ Ru.

The ability of the adsorbent (mixture of activated carbon with zinc and palladium powders) to remove ¹⁰⁶ Ru is not dependent on the pH of the solution that is fed through the column packed with said adsorbent. If the removal efficiency of the adsorbent is reduced, it can be regenerated or activated again by washing it with aqueous nitric acid or water.

Liquid wastes containing ¹⁰⁶ Ru are processed as follows by the method of the present invention: the wastes after pH adjustment are passed through a column packed with an adsorbent comprising a 1:0.01:1 mixture of zinc powder, palladium powder and activated carbon; when the concentration of radioactive ¹⁰⁶ Ru in the effluent from the column is increased as an indication of reduced removal efficiency, the passage of the liquid wastes are stopped, and instead, aqueous nitric acid or water is passed through the column to wash and reactivate the adsorbent; and thereafter, the next portion of the ¹⁰⁶ Ru containing liquid wastes with a properly adjusted pH are passed through the regenerated column. By repeating this cycle, ¹⁰⁶ Ru can be effectively removed from the liquid wastes.

The advantages of the present invention will become more apparent by reading the following Examples and Comparative Examples. The radioactive ruthenium containing feed that was processed by the method of the present invention in Example 1 was liquid wastes resulting from the production of ⁹⁹ Mo by the following procedure: uranium dioxide irradiated in a nuclear reactor was dissolved in nitric acid and ⁹⁹ Mo was extracted from the solution with an organic solvent, and the resulting highly radioactive liquid wastes were neutralized with sodium hydroxide for separation of sodium uranate by filtration. In Examples 2 and 3, as well as in Comparative Examples 1 and 2, group separated liquid wastes were used; high-level radioactive liquid wastes discharged from the spent nuclear fuel reprocessing plant at the Power Reactor and Nuclear Fuel Development Corporation was subjected to extraction of U, Pu and trans-Pu and other radioactive elements with organic solvents, and thereafter, nuclear fission products were separated by passing the liquid wastes through a zeolite or titanic acid packed column. Neither type of liquid wastes contained ¹⁰⁶ Ru in a cationic chemical form that could be readily removed by the coagulation/precipitation technique or ion exchange reaction. In both types of liquid wastes, ¹⁰⁶ Ru was present as various nitrosyl compounds most of which were in the form of anionic forms that could not easily be removed by any of the conventional chemical methods. Both types of liquid wastes contained about 0.4 mol of sodium nitrate and passed through the adsorbent-packed column at a flow rate of about 3 cm/min.

The column used in each of the Examples and Comparative Examples was made of glass and measured 8 mm in inside diameter and 200 mm long; each column was packed with 1.0 g of zinc powder (60-80 mesh), 0.01 g of palladium powder (-100 mesh) and 1.0 g of activated carbon (60-300 mesh).

The performance of the adsorbent was represented by a decontamination factor which was the ratio of the concentration of radioactive ¹⁰⁶ Ru in the feed to that in the effluent.

EXAMPLE 1

Six 50-ml portions of the liquid wastes resulting from the production of ⁹⁹ Mo and containing ¹⁰⁶ Ru were adjusted to pHs in the range of 2.1 to 12.5. Then, the samples were passed through columns packed with the adsorbent comprising a mixture of activated carbon with zinc and palladium powders. The results are shown in Table 1.

                  TABLE 1                                                          ______________________________________                                                  .sup.106 Ru radioactivity level                                       pH of liquid                                                                            (μCi/ml)        Decontamina-                                       wastes feed                                                                             Feed         Effluent  tion factor                                    ______________________________________                                         12.5     5.3 × 10.sup.-2                                                                       undetected                                                                               >10.sup.2                                      11.3     5.3 × 10.sup.-2                                                                       undetected                                                                               >10.sup.2                                      10.3     5.3 × 10.sup.-2                                                                       undetected                                                                               >10.sup.2                                      8.3      5.3 × 10.sup.-2                                                                       undetected                                                                               >10.sup.2                                      3.7      5.3 × 10.sup.-2                                                                       undetected                                                                               >10.sup.2                                      2.1      5.3 × 10.sup.-2                                                                       undetected                                                                               >10.sup.2                                      ______________________________________                                    

The data in Table 1 show that the performance of the column packed with a mixture of activated carbon with zinc and palladium powders was independent of the acidity of the waste liquor that was passed through the column.

Comparative Example 1

Two columns were prepared; one was packed with a mixture of activated carbon with zinc and palladium powders, and the other was packed with a mixture of activated carbon and zinc powder. Varying amounts of group separated waste liquor containing ¹⁰⁶ Ru and adjusted to an alkaline pH (=10.1) with sodium hydroxide were passed through each column. The results are shown in Tables 2 and 3.

                  TABLE 2                                                          ______________________________________                                         Column Packed with Mixture of Zn                                               Powder and Activated Carbon                                                    Amount of liquid                                                                           .sup.106 Ru radioactivity                                          wastes fed  level (μCi/ml)                                                                              Decontamination                                    (ml)        Feed      Effluent  factor                                         ______________________________________                                         30          3.9 × 10.sup.-2                                                                    4.1 × 10.sup.-4                                                                    96                                             64          3.9 × 10.sup.-2                                                                    1.4 × 10.sup.-3                                                                    28                                             98          3.9 × 10.sup.-2                                                                    2.1 × 10.sup.-3                                                                    19                                             120         3.9 × 10.sup.-2                                                                    2.5 × 10.sup.-3                                                                    16                                             190         3.9 × 10.sup.-2                                                                    5.3 × 10.sup.-3                                                                     7                                             ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                         Column Packed with Mixture of                                                  Activated Carbon with Zn and Pd Powders                                        Amount of liquid                                                                          .sup.106 Ru radioactivity                                                                          Decontam-                                       wastes fed level (μCi/ml)   ination                                         (ml)       Feed       Effluent     factor                                      ______________________________________                                          30        3.9 × 10.sup.-2                                                                     ˜1.5 × 10.sup.-5                                                                >10.sup.3                                    64        3.9 × 10.sup.-2                                                                     2.9 × 10.sup.-4                                                                       140                                          98        3.9 × 10.sup.-2                                                                     5.8 × 10.sup.-4                                                                       68                                          140        3.9 × 10.sup.-2                                                                     8.9 × 10.sup.-4                                                                       44                                          200        3.9 × 10.sup.-2                                                                     1.5 × 10.sup.-3                                                                       26                                          240        3.9 × 10.sup.-2                                                                     2.0 × 10.sup.-3                                                                       20                                          ______________________________________                                    

COMPARATIVE EXAMPLE 2

Two columns were prepared; one was packed with a mixture of activated carbon with zinc and palladium powders, and the other was packed with a mixture of activated carbon and zinc powder. Varying amounts of group separated liquid wastes containing ¹⁰⁶ Ru and adjusted to an acidic pH (=2.2) with nitric acid were passed through each column. The results are shown in Tables 4 and 5.

                  TABLE 4                                                          ______________________________________                                         Column Packed with Mixture of                                                  Zn Powder and Activated Carbon                                                 Amount of liquid                                                                          .sup.106 Ru radioactivity                                           wastes fed level (μCi/ml)                                                                               Decontamination                                    (ml)       Feed      Effluent   factor                                         ______________________________________                                         26         3.9 × 10.sup.-2                                                                    2.5 × 10.sup.-4                                                                     160                                            58         3.9 × 10.sup.-2                                                                    3.3 × 10.sup.-4                                                                     120                                            96         3.9 × 10.sup.-2                                                                    2.8 × 10.sup.-4                                                                     140                                            130        3.9 × 10.sup.-2                                                                    2.8 × 10.sup.-4                                                                     140                                            150        3.9 × 10.sup.-2                                                                    4.8 × 10.sup.-4                                                                      82                                            ______________________________________                                    

                  TABLE 5                                                          ______________________________________                                         Column Packed with Mixture of                                                  Activated Carbon with Zn and Pd Powders                                        Amount of liquid                                                                          .sup.106 Ru radioactivity                                           wastes fed level (μCi/ml)                                                                               Decontamination                                    (ml)       Feed      Effluent   factor                                         ______________________________________                                          26        3.9 × 10.sup.-2                                                                    4.2 × 10.sup.-5                                                                     940                                             58        3.9 × 10.sup.-2                                                                    1.7 × 10.sup.-4                                                                     230                                            100        3.9 × 10.sup.-2                                                                    2.4 × 10.sup.-4                                                                     160                                            130        3.9 × 10.sup.-2                                                                    2.7 × 10.sup.-4                                                                     150                                            150        3.9 × 10.sup.-2                                                                    4.5 × 10.sup.-4                                                                      87                                            ______________________________________                                    

Whether the liquid wastes to be treated were acidic or alkaline, the adsorbent according to the present invention that was comprised of a mixture of activated carbon with zinc and palladium powders was more effective in ¹⁰⁶ Ru removal than the adsorbent consisting of a mixture of activated carbon and zinc powder. The advantage of the addition of palladium powder was particularly obvious in the alkaline waste liquor.

EXAMPLE 2

The column that was packed with a mixture of activated carbon with zinc and palladium powders and through which varying amounts of group separated liquid wastes in Comparative Example 2 were passed washed by passage of 30 ml of water. Thereafter, varying amounts of group separeted liquid wastes adjusted to pH 2.8 with nitric acid were passed through the same column to examine whether the column could be regenerated by washing with water. The results are shown in Table 6.

                  TABLE 6                                                          ______________________________________                                         Amount of liquid                                                                          .sup.106 Ru radioactivity                                           wastes fed level (μCi/ml)                                                                               Decontamination                                    (ml)       Feed      Effluent   factor                                         ______________________________________                                         25         3.9 × 10.sup.-2                                                                    4.3 × 10.sup.-5                                                                     910                                            50         3.9 × 10.sup.-2                                                                    2.1 × 10.sup.-4                                                                     190                                            75         3.9 × 10.sup.-2                                                                    4.6 × 10.sup.-4                                                                      85                                            100        3.9 × 10.sup.-2                                                                    4.6 × 10.sup.-4                                                                      85                                            ______________________________________                                    

EXAMPLE 3

Varying amounts of group separated liquid wastes containing ¹⁰⁶ Ru and adjusted to pH 8.5 with sodium hydroxide were passed through a column packed with a mixture of activated carbon with zinc and palladium powders. Thereafter, the column was washed with 30 ml of aqueous nitric acid (pH: 2.2), and again fed with varying amounts of group separated liquid wastes that had been adjusted to pH 7.6. The effectiveness of aqueous nitric acid in regenerating the deactivated column will be apparent from the following Tables 7 and 8.

                  TABLE 7                                                          ______________________________________                                         Initial Pass                                                                   Amount of liquid                                                                          .sup.106 Ru radioactivity                                           wastes fed level (μCi/ml)                                                                               Decontamination                                    (ml)       Feed      Effluent   factor                                         ______________________________________                                         25         3.9 × 10.sup.-2                                                                    9.4 × 10.sup.-5                                                                     420                                            50         3.9 × 10.sup.-2                                                                    9.9 × 10.sup.-4                                                                     40                                             75         3.9 × 10.sup.-2                                                                    8.7 × 10.sup.-4                                                                     45                                             100        3.9 × 10.sup.-2                                                                    1.1 × 10.sup.-4                                                                     36                                             ______________________________________                                    

                  TABLE 8                                                          ______________________________________                                         Second Pass after Washing                                                      Amount of liquid                                                                          .sup.106 Ru radioactivity                                           wastes fed level (μCi/ml)                                                                               Decontamination                                    (ml)       Feed      Effluent   factor                                         ______________________________________                                         25         3.9 × 10.sup.-2                                                                    1.3 × 10.sup.-4                                                                     300                                            50         3.9 × 10.sup.-2                                                                    5.0 × 10.sup.-4                                                                     80                                             75         3.9 × 10.sup.-2                                                                    5.1 × 10.sup.-4                                                                     77                                             100        3.9 × 10.sup.-2                                                                    4.6 × 10.sup.-4                                                                     85                                             ______________________________________                                    

The results in Examples 2 and 3 show that a deactivated adsorbent comprising a mixture of activated carbon with zinc and palladium powders could be effectively regenerated by washing with water when the adsorbent was used to remove ¹⁰⁶ Ru from an acidic radioactive liquid waste, whereas washing with aqueous nitric acid proved effective when an alkaline liquid wastes were fed through the column.

By reading the foregoing description, it will be understood that the present invention has the following advantages: (1) the claimed method utilizes both the adsorbing action of activated carbon and the electrochemical action that occurs between the carbon palladium electrode and the zinc electrode in the liquid electrolyte liquid wastes and as a result, the method is capable of removing nitrosyl ruthenium compounds that have been difficult to eliminated by the conventional techniques, and this contributes to a higher efficiency in ¹⁰⁶ Ru removal; (2) the ability of the adsorbent to remove ¹⁰⁶ Ru is not dependent on the pH of the liquid wastes to be passed through the column; and (3) if the ability of the adsorbent to remove ¹⁰⁶ Ru becomes reduced, it can be reactivated by washing with aqueous nitric acid or water. 

What is claimed is:
 1. A method of processing radioactive liquid wastes containing radioactive ruthenium by passing said wastes having pH of about 2 to about 12 through a column packed with an adsorbent comprising a 1:1:0.01 mixture of activated carbon, zinc and palladium powders.
 2. A method according to claim 1 which further includes the step of regenerating a deactivated adsorbent by passing aqueous nitric acid or water through the column containing such deactivated adsorbent. 